1. Field of the Invention
The technology disclosed relates to a controller and a control method for a motorised vehicle, and in particular to the control of a motorised vehicle having at least two driven wheels driven independently by a motor arrangement. In accordance with such motorised vehicles, turning (or spinning) of the vehicle is typically effected by driving the at least two driven wheels in opposite directions. A typical example of such a motorised vehicle is an electric wheelchair.
2. Background of the Invention
Conventional electric wheelchairs have at least two independently driven wheels. Whilst the motor arrangement used to drive the wheels may comprise a single motor with independent couplings between the motor and the wheels to effect independent drive of those wheels, it is more common that each driven wheel is driven by an independent motor. Often the electric wheelchair will include two or more castors which rotate to follow the direction of travel, and optionally a number of further fixed wheels may be provided. The speed and direction of each driven wheel defines a linear forward/reverse speed and the turning rate of the wheelchair.
A typical input control interface for an electric wheelchair provides a joystick having two control axes, one to control the linear/reverse speed and one to control the turn rate (or spin speed). However, it will be appreciated that a joystick is not a requirement, and many other types of input control interface have been developed to allow control by users with a variety of different disabilities, for example head control based mechanisms, sip puff based mechanisms, etc.
The use of ‘IR compensation’ is common in controllers for motorised vehicles such as electric wheelchairs. IR compensation can be thought of as ‘estimated speed control’, which generally provides better control of the wheelchair when compared to an open loop system. However, the technique is not perfect and suffers from a number of drawbacks.
In accordance with IR compensation techniques, the speed of the motor is estimated using the following equation:Speed=kemf(Vm−Im×Rm)where:Kemf=Back emf constantVm=voltage applied to motorIm=Current drawn by motorRm=Resistance of motor windings
The value of Rm is programmed to a chosen value, and then the current drawn by the motor is monitored. As the current varies the voltage applied to the motor is varied with the aim of keeping the speed constant whilst the user input is indicating a constant speed should be maintained.
A significant drawback is that the motor resistance can vary significantly under normal use (typically due to manufacturing tolerances and variation in operating temperature), and the programmable setting of Rm has to be set to a value low enough to take those variations into account. If this is not done, the speed of the motor can become unstable due to the IR compensation technique, leading to juddering and general instability of the wheelchair.
This intentionally low value of the programmed Rm causes an error in the speed estimation under normal circumstances and this can be particularly significant at low speeds. The end result could be a wheelchair failing to move at all when the speed control is set low, which is disconcerting for the user. The problem occurs most when the motors are under load and this can be due to the type of surface (e.g. thick carpet), the castor assembly design, the user weight, an obstacle, etc. The turn (or spin) axis is particularly susceptible to this problem, as realignment of the castors can require significant torque.
Due to the desire to produce motorised vehicles such as electric wheelchairs as cheaply as possible, it is common to use low cost components such as low cost motors. The above discussed problems become even more significant when low cost motors are used, since such motors usually have high resistance windings that heat up under normal use and then have an even higher resistance.
It would hence be desirable to provide an improved technique for controlling motorised vehicles that alleviates the above-discussed problems.